3. <meta charset=…>definition is highly recommended right after <!DOCTYPE… or at the top of <head> section. UTF-8 is the most widely used encoding (most compact and fastest for pretty much all browsers).

4. Natural language specification, e.g. lang=”en”is optional, but recommended, as it allows search engines and screen readers to identify section correctly. Specifying language for html tag defines it for entire page, however it also can be specified for elements, such as div.

5. Stylesheet definition does not require type=”text/css” specification, since “rel” (relationship) defines link as stylesheet, and css is the only style sheet language.

6. Similarly, no need to specify language=”JavaScript” for script tag: JavaScript is assumed by default.

7. Closing </script> tag is required, even when referring an external JavaScript file.

8. Comment <!– saved from url=(0014)about:internet –> is only required to enable debugging of local page in IE; if not specified, when you load local page, IE prevents running any JavaScript on the page, unless you explicitely click “Allow blocked content” (this comment is ignored by any other browser)

9. Tags can be lower and/or upper case.

10. Attributes can be specified without values (e.g. <input type=”checkbox” checked>). This is not commonly used, however, since inconsistent with other attributes. XHTML standard required value for each attribute (e.g. <input type=”checkbox” checked=”checked” />).

12. Quotation marks around attribute values are only required if attribute values contain >, = or space. E.g. “<img alt=”This is an image” src=image.gif> is valid for HTML5, but not XHTML.

Loose rules 9-12 are likely to fail on validators. It also may be considered a bad “not-so-clean” style. From the browser perspective, however, those inconsistencies do not add any overhead, and thus following XHTML rules is not necessary from performance or compatibility perspective.